Mr Almy, On a Diminution of the Potential Difference , etc. 1 83 
On a Diminution of the Potential Difference between the Elec- 
trodes of a Vacuum Tube, produced by a Magnetic Field at the 
Cathode. By John E. Almy, University of Nebraska. 
[Read 6 May 1901.] 
Characteristic effects of a magnetic field upon the discharge 
in vacuum tubes, especially with large field strength at the 
cathode have been noted and studied by Birkeland, Melani 1 , and 
others. Birkeland found that with the lines of force parallel to 
the direction of the discharge, at gas pressures below 0012 mm. 
of mercury, the P.-D. between the electrodes was made to diminish 
by the magnetic field, at first rather slowly as the magnetic in- 
tensity increases ; when however a certain “ critical intensity ” is 
reached a large, abrupt, increase of this diminution is obtained 
with a very small increase of magnetic intensity. Further study 
of this effect was my purpose. 
I found that not only fields parallel to the discharge, but also 
a field normal to the direction of discharge, gave this large 
and rather anomalous diminution of potential difference, and in 
fact the so-called “ critical ” intensity was much smaller in the 
latter case. 
Using an induction coil giving a 6-inch spark to furnish the 
discharge, a spark gap, with two brass spheres in shunt to the 
tube to measure the potential difference between electrodes, 
a solenoidal electro-magnet with soft iron core 4 cm. in diameter, 
about 40 cm. long, to give the magnetic fields, the effects pro- 
duced in a tube 3*5 cm. in diameter, 20 cm. in length, having 
plane aluminium electrodes (discs), were observed. The following 
series, with gas pressure of 0'09 mm. of mercury, are typical : 
1 Cf. Birkeland, Comptes Rendus, Yol. 126, p. 586 ; Melani, Nuovo Cimento 
(Series 4), Yol. 5, p. 329. 
